And while about 2,000 American troops are now based in the civil war-torn country, that number could increase before a final pull-out, according to the New York Times.
"Will devastate Turkey economically if they hit Kurds", Trump continued.
"We also know that those fighting alongside us for all this time deserve to be protected as well", he added.
No U.S. personnel have left Syria yet, according to a Pentagon spokesman.
The announcement of a pullout is fueling concern about how quickly the US will abandon its Kurdish allies, amid contradictory statements recently by Trump administration officials on an exit timetable.
Senior US officials were shocked too, among them Defense Secretary Jim Mattis, who quit in protest.
The previous week, Trump's National Security Advisor John Bolton had announced that Washington would withdraw its troops from Syria only after defeating ISIS and ensuring that the Kurds are safe.
The US decision has injected new uncertainty into the eight-year-old Syrian war and spurred a flurry of contacts over how the security vacuum will be filled in the swathe of northern and eastern Syria where the US forces are now stationed.
President Donald Trump on Sunday tweeted that the "long overdue pullout from Syria" is starting, and threatened that the us will "devastate Turkey economically" if they attack Kurdish forces in the war-torn country.
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Residents near border crossings that are typically used by United States forces going in and out of Syria from Iraq said they had seen no obvious or large-scale movement of USA ground forces on Friday.
"Likewise, do not want the Kurds to provoke Turkey".
Erdogan said Bolton was making a "serious mistake".
President Trump's unexpected announcement last month that troops would leave the country has fueled fears that a power vacuum could strengthen ISIS and cede influence in the region to Iran and Russian Federation.
His top diplomat Mike Pompeo is on a whirlwind regional tour aimed at reassuring Washington's Kurdish allies in the fight against Isis.
He said the United States envoy for Syria, Jim Jeffrey, had travelled to northeast Syria this week and would soon go to Ankara for talks, including on moving forward a UN-led political process to end the eight-year long conflict in Syria.
Maria Zakharova, a Russian Foreign Ministry spokeswoman, said it was important for Syrian Kurds and the Syrian government to start talking to each other in light of the U.S. withdrawal plans.
Trump's comments follow Turkish President Recep Tayyip Erdogan's pledge to take action against the YPG, which Turkey considers a terrorist organization linked to the outlawed Kurdish Workers' Party (PKK) within Turkey.